Re: [uml-devel] Swapping directly to host swap area?

[BlaisorBlade <blaisorblade_spam@yahoo.it>]

Alle 05:39, mercoledì 28 gennaio 2004, Jeff Dike ha scritto:

> > In the current setup, the host kernel sees this as a normal
> > write to an open file, caching it in the process and so not truly
> > freeing up the ram for a bit.
>
> This is, and this could be done by setting O_DIRECT on UML swap devices. 
> That way, writes go directly (and asynchronously) to the device without
> landing in the cache.
>
> With mmap in effect, this can also be done with a MADVISE_DONTNEED on the
> relevant pages of the swap file.
>
> Patches cheerfully accepted.

About this, my idea was that using swap both on the host and on the guests is 
not very useful:

- because the host already sees which pages are accessed and which not (an 
access inside UML is an access inside the host); the only difference is that 
the host can swap even the guest kernel, while UML not;

- because that probably creates overhead: page aging probably has little one 
(it should be possible to disable it with CONFIG_SWAP=n), but since UML (if I 
understand correctly) needs to trap a SIGSEGV (which is slow, according to 
what you say about TT and SKAS) from a page access to swap in a page, while 
the host does this in hardware, my opinion is that you would probably disable 
swap inside UML. Swap on the host means just that the host will cache datas 
instead of keeping in memory UML pages, if you don't give to the guests more 
than the physical memory; but this makes sense, IMHO. When you have different 
customers on the same host, there is a problem, but then they can choose 
differently (even because this doesn't constrast with the changes to code 
suggested above).

I think this is correct even when /dev/anon is used, but I'm not very sure for 
it.
-- 
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Paolo Giarrusso, aka Blaisorblade
Linux Kernel 2.4.23/2.6.0 on an i686; Linux registered user n. 292729
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